Unconventional superconductivity which possesses non-trivial topological properties resulting in a state with broken time-reversal symmetry is extensively sought for in the modern field of quantum materials. Theoretically, spontaneous TRS-breaking arises naturally from fundamental symmetry considerations. Potential applications to topological quantum computation have placed such materials at the forefront of experimentation: search for such phases of matter remains a major direction in the contemporary study of superconductivity.
Although recent polar Kerr effect studies have identified spontaneous time-reversal symmetry breaking in a number of unconventional superconductors, difficulties with interpretation rose because all the measurements were restricted to a single near-visible radiation frequency, which is more than three orders of magnitude higher than relevant to unconventional superconductivity energy scale, typically at 0.1THz. Consequently, the results brought considerable interpretation debates: e.g. is this a bulk property of the superconductor, or does the signal being influenced by inhomogeneities and is not representative of the sample as a whole? More importantly, the microscopic origin and quantitative understanding of the time-reversal-symmetry-broken state remains topic of ongoing disputations that can only be resolved by ultra-low-frequency spectroscopic measurements of Kerr angle rotation as function of frequency in the window relevant to superconductivity and magnetism that originates the time-reversal-symmetry-breaking effect (development of a spontaneous magnetic moment). Thus, new spectroscopic methods as probes of strong electron correlations are required.
The developed in this project one-of-a-kind apparatus for the magneto-optical polar Kerr angle spectroscopy at sub-THz frequency – the new diagnostics method with wide-ranging transformative impact – brings to light the opportunities to study optical Hall conductivity at ac frequencies, the broken time-reversal symmetry in unconventional superconductors, the origin of unconventional pairing, the in-gap collective modes, and the structures of the superconducting order parameters. These studies will remain in the focus till the end of this project.
